Aliso: A Journal of Systematic and Evolutionary Botany Volume 23 | Issue 1 Article 23 2007 Plasticity of Chasmogamous and Cleistogamous Reproductive Allocation in Grasses Gregory P. Cheplick College of Staten Island, City University of New York, Staten Island Follow this and additional works at: http://scholarship.claremont.edu/aliso Part of the Botany Commons, and the Ecology and Evolutionary Biology Commons Recommended Citation Cheplick, Gregory P. (2007) "Plasticity of Chasmogamous and Cleistogamous Reproductive Allocation in Grasses," Aliso: A Journal of Systematic and Evolutionary Botany: Vol. 23: Iss. 1, Article 23. Available at: http://scholarship.claremont.edu/aliso/vol23/iss1/23 Aliso 23, pp. 286–294 ᭧ 2007, Rancho Santa Ana Botanic Garden PLASTICITY OF CHASMOGAMOUS AND CLEISTOGAMOUS REPRODUCTIVE ALLOCATION IN GRASSES GREGORY P. CHEPLICK Department of Biology, College of Staten Island, City University of New York, Staten Island, New York 10314, USA ([email protected]) ABSTRACT Cleistogamy is more common in grasses than in any other angiosperm family. Both self-fertilized cleistogamous (CL) spikelets and open-pollinated chasmogamous (CH) spikelets are typically pro- duced. Relative allocation to CL and CH varies among species and populations, and is influenced by ontogeny and environment. The balance between reproductive modes can be expressed as a CH/CL ratio. This ratio is very plastic and stressful conditions can result in values Ͻ1.0. In Amphicarpum purshii, an annual with subterranean CL spikelets, CH/CL declined as density increased because CH decreased more than CL as size was reduced by intraspecific competition. In the shade-tolerant annual Microstegium vimineum, CH/CL was lowest in large greenhouse-grown plants in an unlimited, sunny environment, but was highest in small plants from a shady forest interior; tiller vegetative mass often showed a negative allometric relation to CH and CL allocation. In the perennial Dichanthelium clan- destinum, CH and CL allocation varied among populations, but there was no consistent effect of light on CH/CL. The phenology of reproduction strongly affects CH/CL. In Danthonia spicata, CH/CL was high early in the season as CH flowering commenced, but dropped quickly as axillary CL spikelets matured; A. purshii showed the opposite pattern because CL reproduction occurred first. The assump- tion that cleistogamy simply provides reproductive assurance should be reevaluated in light of new information on phenology and allometry. Changes in the balance between CH and CL caused by environmental factors may be indirect effects of size. Evolutionary models that do not explore the plasticity and allometry of CH and CL reproduction may not be useful in predicting the myriad patterns in nature. Key words: allometry, chasmogamy, cleistogamy, grasses, phenotypic plasticity, Poaceae, reproduc- tive allocation. INTRODUCTION of CL may be difficult to detect in the field and have prob- ably led to underestimates of the extent of self-fertilization Cleistogamy (CL) is the production of flowers that do not in grasses. Other types of CL are more readily observed by open and undergo self-fertilization ‘‘in the bud’’ (Lord casual inspection. ‘‘Sheath fertilization’’ involves the reten- 1981). Most species that have CL flowers can and do con- tion of spikelets on non-emergent, axillary inflorescences en- tinue to make open, potentially cross-pollinated chasmoga- closed within leaf sheaths and is the most common type of mous (CH) flowers. Because an individual usually can make CL in grasses (Campbell et al. 1983). Depending on the both floral forms and they may be present at the same time, species, axillary spikelets may be found all along the stem CL species are often considered to have a mixed breeding axis or be mostly concentrated at the basal nodes (Dykster- system or ‘‘multiple strategy’’ (Lloyd 1984; Schoen 1984; huis 1945; Dobrenz and Beetle 1966; Cheplick and Clay Redbo-Torstensson and Berg 1995; Masuda et al. 2001). Due 1989; Cheplick 1996). In a few grasses, CL spikelets and to the structural and developmental differences typically found between CH and CL flowers (and sometimes, between caryopses (seeds) mature at the ends of specialized culms the resulting fruits and seeds), CL species are also consid- beneath the soil surface (Campbell et al. 1983; Cheplick ered to show reproductive dimorphism (Lord 1981; Clay 1994). 1983a; Schoen and Lloyd 1984; Cheplick 1994; Plitmann The relative extent of CL can vary greatly both between 1995). and within phylogenetically related species (Campbell In Poaceae, CL is more common than in any other angio- 1982). Perhaps the best example is Clay’s (1983a) detailed sperm family and has been reported in over 320 species comparison of multiple populations of four species of Dan- (Campbell et al. 1983). Cleistogamy may be due to spikelets thonia DC. Percent CL was defined as the number of CL that do not open because of modified floral parts, precocious spikelets divided by the number of CH ϩ CL spikelets. maturation while retained within enclosing leaf sheaths, or Hence, % CL shows the fraction of the total flowers that lodicule failure (Campbell 1982; Heslop-Harrison and Hes- were CL at the time the population was sampled. For five lop-Harrison 1996). For example, Campbell (1982) de- populations of D. sericea Nutt., mean % CL (Ϯ SE) was scribed how, in some species of Andropogon L., spikelets only 2.7 Ϯ 0.3%, while for two populations of D. compressa within the raceme sheath can go through anthesis and self- Austin % CL was 50.2 Ϯ 1.0%. Populations were most var- fertilize before the inflorescence fully emerges from the en- iable for D. spicata (L.) P. Beauv., ranging from 5 to 43% closing sheath. Without detailed observation, the above types CL (x Ϯ SE ϭ 24.5 Ϯ 0.4%). Because higher % CL in D. VOLUME 23 Chasmogamy/Cleistogamy in Grasses 287 spicata was associated with disturbed and mown habitats; grasses (Clay 1982, 1983a; Bell and Quinn 1987; Cheplick Clay (1983a) suggested grazing pressure might generally fa- 1995). Environmental factors known to affect the extent of vor increased levels of CL in grasses, a view shared by oth- CL in grasses include light, soil nutrition and moisture, and ers (Dyksterhuis 1945; Campbell et al. 1983). competition and herbivory. Because CL flowers are energy Other research has indicated that the extent of CL in efficient for a plant to produce and provide reproductive as- grasses can vary among seed-derived sibships (Clay 1982; surance (Schemske 1978; Schoen 1984; Schoen and Lloyd Cheplick and Quinn 1988) and among clonal replicates in 1984), it has long been noted that whenever environmental perennials (Cheplick 1995). In Amphicarpum purshii Kunth, conditions are stressful there tends to be greater reproduction an annual, the ratio of seeds from CH spikelets to those from by CL compared to CH (Dyksterhuis 1945; Wilken 1982; CL spikelets had a significant narrow-sense heritability of Campbell et al. 1983). Hence, the ratio of CH to CL is ex- 0.56 (Cheplick and Quinn 1988). For the perennial Dan- pected to decline with increasing environmental stress. It is thonia spicata, Clay (1982) reported broad-sense heritabili- also recognized that both CH and CL depend on ontogeny, ties of 0.53 and 0.72 in the field and greenhouse, respec- varying with plant size and phenological stages in ways that tively, for the proportion of CL spikelets produced per tiller. may be predictable for some species (Clay 1982; Jasieniuk Although simple recessive inheritance of CL (one to three and Lechowicz 1987; Cheplick and Clay 1989; Cheplick genes) has been reported in some grain crops (Merwine et 1994; Diaz and Macnair 1998). As will be explored later in al. 1981; Chhabra and Sethi 1991; Kurauchi et al. 1993; this paper, the challenge for the future generation of models Ueno and Itoh 1997), like most life-history traits in wild of the evolution of mixed CH/CL breeding systems will be species, CL is likely to be under quantitative genetic control to account for the marked plasticity of the two reproductive (Mazer and LeBuhn 1999). Given the primacy of genetic modes in relation to environmental factors. In addition, they variation to the microevolution of populations, it is surpris- will need to explore the relations of plant size and pheno- ing that so little effort has gone into documenting quantita- logical stage to observed changes in the extent of CL shown tive genetic variation in the extent of CL vs. CH in different during plant development. species. Objectives Theory Given the predictions made by theoretical models for the Theoretical considerations of the evolution of CL have evolution of CH/CL and the complexities inherent in trying involved comparative cost-benefit analyses of CH vs. CL to understand life-history traits that vary greatly with envi- flower production (Schoen and Lloyd 1984) or variation in ronmental conditions, one objective of this paper will be to the relative fertility of the two floral types (Redbo-Torstens- summarize information on the relative allocation to CH and son and Berg 1995; Masuda et al. 2001). Because the pa- CL in a variety of grasses exposed to variable environments. ternal costs of producing prodigious amounts of pollen in The balance between reproductive modes will be expressed outcrossed spikelets may be high in grasses (e.g., Schoen as a CH/CL ratio. Because many of the changes in this bal- 1984), maternal reallocation of conserved resources to CL ance that are mediated by environmental factors may be the spikelets could enhance plant fitness by increasing the num- indirect effect of plant size, a second objective will be to ber or size of maturing seeds (Schoen and Lloyd 1984; present allometric relationships of CH and CL to vegetative Lloyd 1987). Indeed, seeds of CL spikelets are heavier than mass for several grass species for which data are available.